I can't think of an episode of TNG, or any episode from Trek, that was actually scary. "Night Terrors" was probably as close as they got to making a real sci-fi thriller story. I suppose ENT's "Impulse" could be seen as a zombie-like thriller.

Night Terrors definitely had it's scaryish moments, for me the thing with Identity Crisis was more how much it creeped me out rather than being scary per se (I don't generally get scared by stuff anyway tbh, most horror films do nothing for me, but I get creeped out by things!)